1. Field of the Invention
The present invention relates to an implantable microphone device, and in particular, to such a device suitable for use with an implanted hearing prosthesis.
2. Related Art
Hearing prostheses of various types are widely used to improve the lives of users. Such devices include, for example, hearing aids, cochlear implants, middle ear implants and electro-acoustic devices. A current trend is to develop totally implantable forms of these devices. Totally implantable devices have the advantage of allowing the user to have a superior aesthetic result, as the user is visually indistinguishable in day to day activities. They have a further advantage in generally being inherently waterproof, allowing the user to shower, swim, and so forth without needing to take any special measures.
Conventional hearing prostheses, for example partially implanted cochlear implant systems, use externally disposed microphones. To provide a totally implantable prosthesis a suitable implantable microphone needs to be employed. Replacing the external microphone assembly with a subcutaneous microphone assembly presents various practical difficulties. Principally, an implantable microphone assembly needs to be hermetically sealed.
The purpose of a microphone is to measure pressure variations in an audible frequency range. A conventional microphone includes a sound capturing membrane. To measure such pressure variations, the differential pressure between a side of the membrane, from where the sound originates, and the opposing reference side is measured. A conventional microphone also includes a purge hole disposed on the reference side of the membrane which exposes the reference side to outside ambient pressure. The purge hole is used to keep the ambient pressure on the reference side equal with the outside ambient pressure. The purge hole is necessary to compensate for any slow ambient pressure variations which could affect the measurement of differential pressure and hence the quality of the microphone output.
Given that an implantable microphone assembly needs to be hermetically sealed, it is clear that the use of such a purge hole cannot be employed.
In U.S. Pat. No. 7,322,930 there is disclosed an example of an implantable microphone device in which a microphone assembly is provided in a sealed cavity. The microphone assembly is provided with a flexible membrane which is intended to increase the sensitivity of the device. In practice, however, the quality of the microphone output has been found to be limited.
The issue of providing a microphone without an exterior purge hole is also encountered in hydrophones. U.S. Pat. No. 4,868,799 discloses an example of a hydrophone arrangement which deals with the issue by way of active pressure compensation. A valve operated mechanism increases the internal pressure to compensate for increasing external pressure depending upon the depth the hydrophone is submerged. While this may prove to be an effective solution where size of the device is not of concern, in respect of an implantable device such a bulky mechanism is impractical.